Large electric motors and generators having rotors which are driven at high speeds present problems of structural integrity. It is important in the design of such machines to consider carefully the method of joining together materials in the rotor because of the large centrifugal forces acting at the high speeds of rotation encountered in some machines, such as those having turbine driven rotors. The use of bolts or rivets results in high unit stresses requiring very large bolts or rivets to provide the necessary holding strength. The large holes in the pole pieces to accommodate the bolts or rivets disrupts the symmetry and reduces the efficiency of magnetic operation.
It has been a common practice to fabricate high speed rotors by welding together separately fabricated parts to avoid the disadvantages of bolted or riveted assemblies. The use of welding, however, introduces other problems. To achieve structural integrity in welded assemblies the welding must be done with extreme care and the structure must be designed so that a practical weld depth is maintained. These considerations generally result in a manufacturing process of many steps involving large amounts of time and labor.